Can we figure out how much the Sun really affects propagation?

Foundations of Amateur Radio

Recently I saw a social media post featuring a screenshot of some random website with pretty charts and indicators describing "current HF propagation". Aside from lacking a date, it helpfully included notations like "Solar Storm Imminent" and "Band Closed".

It made me wonder, not for the first time, what the reliability of this type of notification is. Does it actually indicate what you might expect when you get on air to make noise, is it globally relevant, is the data valid or real-time? You get the idea.

How do you determine the relationship between this pretty display and reality?

Immediately the WSPR or Weak Signal Propagation Reporter database came to mind. It's a massive collection of signal reports capturing time, band, station and other parameters, one of which is the Signal To Noise ratio or SNR.

If the number of sun spots, or a geomagnetic index change affected propagation, can we see an effect on the SNR?

Although there's close on a million records per day, I'll note in advance that my current approach of taking a daily average across all reports on a specific band, completely ignores the number of reports, the types and direction of antennas, the distance between stations, transmitter power, local noise or any number of other variables.

Using the online "wspr.live" database, looking only at 2024, I linked the daily recorded WSPR SNR average per band to the Sun Spot Numbers and Geomagnetic Index and immediately ran into problems. For starters the daily Sun Spot Number or SSN, from the Royal Observatory in Belgium does not appear to be complete. I'm not yet sure why.

For example, there's only 288 days of SSN data in 2024. Does this mean that the observers were on holiday on the other 78 days, or was the SSN zero? Curiously there's 60 days where there's more than one recording and as a bonus, on New Years Eve 2024, there's three recordings, all with the same time stamp, midnight, with 181, 194 and 194 sun spots, so I took the daily average. Also, I ignored the timezone, since that's not apparent.

Similarly the Geomagnetic Index data from the Helmholtz Centre for Geosciences in Potsdam, Germany has several weird artefacts around 1970's data, but fortunately not within 2024 that I saw. The data is collected every three hours, so I averaged that, too.

After excluding days where the SSN was missing, I ran into the next issue, my database query was too big, understandable, since there are many reports in this database, 2 billion, give or take, for 2024 alone.

Normally I'd be running this type of query on my own hardware, but you might know that I lost my main research computer last year, well, I didn't lose it as such, I can see it from where I am right now, but it won't power up. Money aside, I've been working on it, but being unceremoniously moved from Intel to ARM is not something I'd recommend.

I created a script that extracted the data, one day at a time, with 30 seconds between each query. Three hours later I had preliminary numbers.

The result was 6,239 records across 116 bands, which of course should immediately spark interest, since we don't really have that many bands. I sorted the output by the number of reports per band and discovered that the maximum number of days per band was 276. This in turn should surprise you, since there's 365 days in a year, well technically a smidge more, but for now, 365 is fine, not to mention that 2024 was a leap-year.

So, what happened to the other 90 days? We know that 78 are missing because the SSN wasn't in the database but the other 12 days? I'm going to ignore that too.

I removed all the bands that had less than 276 reports per day, leaving 17 bands, including the well known 13 MHz band, the what, yeah, there's a few others like that.

I removed the obvious weird band, but what's the 430 MHz band, when the 70cm band in WSPR is defined as 432 MHz?

I manually created 15 charts plotting dates against SNR, SSN, Kp and ap indices. Remember, this is a daily average of each of these, just to get a handle on what I'm looking at.

Immediately several things become apparent. There are plenty of bands where the relationship between the average SNR and the other influences appear to be negligible.

We can see the average SNR move up and down across the year, following the seasons - which raises a specific question. If the SNR is averaged across the whole planet from all WSPR stations, why are we seeing seasonal variation, given that while it's Winter here in VK, it's Summer on the other side of the equator?

If you compare the maximum average SNR of a band against the minimum average SNR of the same band, you can get a sense of how much the sun spots and geomagnetic index influences the planet as a whole on that band. The band with the least amount of variation is the 30m band.

Said differently, with all the changes going on around propagation, the 30m band appears to be the most stable, followed by the 12m and 15m bands. The SNR across all of HF varies, on average, no more than 5 dB.

The higher the band, the more variation there is. Of course it's also possible that there's less reports there, so we might be seeing the impact of individual station variables more keenly.

It's too early for conclusions, but I can tell you that this gives us plenty of new questions to ask.

I'm Onno VK6FLAB